1. Field of the Invention
The invention relates to a power plane of a multi-layer printed circuit board and more particularly, to power plane regions disposed on multi-layer printed circuit boards.
2. Description of the Prior Art
Generally, printed circuit boards (PCBs) are widely used to electrically connect electrical devices together for transmitting signals. In the past, printed circuit boards were usually single layer. Typically, a surface of the printed circuit board has only a single signal plane. Traces regions on the signal plane are used to connect all electrical devices mounted on the signal plane, and transmit input or output signals of electrical devices. When there is a plurality of different signals among electrical devices, the signal plane needs a complex layout to smoothly transmit a huge volume of signals. However, the size of a printed circuit board and the width and length of traces regions are limited. Limited size of the PCB limits the quantity of the trace regions. For this reason, the quantity of the trace regions is limited on the single layer of PCB having a signal plane. This limitation is not conducive to complex electrical circuits. Thus multi-layer printed circuit board technology has been developed by the PCB industry to increase the total amount of signal planes.
A multi-layer printed circuit board uses an insulating layer to separate two conducting layers. Because a total size of a signal layer (both conducting layers) is increased, a quantity of mounted trace regions on the signal layer is consequently increased to connect more electrical devices and to properly define signal transmission routes among electrical devices. Accordingly, the associated semiconductor processes are improved. As well, the size of integrated circuits (ICs) is greatly reduced. An IC can include more electrical devices to achieve a plurality of different functions. As the multi-layer PCB offers more possibilities for input and output locations to an IC and more flexible signal transmission routes, more complicated ICs having greater functionality can be accommodated. Thus, we can construct a multi-layer printed circuit board to define desired trace regions.
A multi-layer printed circuit board includes plural conducting layers. The outsides of conducing planes are signal planes that include trace regions and mounting pads. The insides of the conducing planes are used as a power plane region and a ground plane for the multi-layer printed circuit board. The power plane region provides a working voltage for electrical devices on the multi-layer printed circuit board. The ground plane provides a ground reference level. Please refer to FIG. 1, FIG. 2, and FIG. 3. FIG. 1 is a diagram of a signal plane 10 of a multi-layer printed circuit board according to the prior art. FIG. 2 is a diagram of a ground plane 20 of the multi-layer printed circuit board according to the prior art. FIG. 3 is a diagram of a power plane region 30 of the multi-layer printed circuit board according to the prior art. The signal plane 10 includes plural trace regions 12, plural mounting pads 14a–14d, and plural vias (or called openings ) 16a–16d. The mounting pads 14a–14d are used to connect to an electrical device 18 (such a IC). The trace regions 12 are used to electrically connect with the mounting pads 14a–14d for transmitting signals. The vias 16a–16d are used to electrically connect the signal plane 10, the ground plane 20, and the power plane region 30. In addition, the ground plane 20 includes plural vias 26b–26d. Surface 22 is a conducting metal layer disposed on the ground plane 20. The power plane region 30 includes plural power blocks 32a–32c. Each block is used to provide different voltage levels for supporting the electrical device 18. For example, the power block 32a provides a voltage level of 1 volt, the power block 32b provides a voltage level of 2 volts, and the power block 32c provides a voltage level of 3 volts.
Following is a description of the operation of the multi-layer PCB with reference to the signal plane 10, the ground plane 20, and the power plane region 30. From top to bottom, the positional relationship of each plane is the signal plane 10, the ground plane 20, and the power plane region 30. If the electrical device 18 must be provided a ground voltage from the mounting pad 14a, the mounting pad 14a must electrically connect with ground plane 20. So the trace regions 12 of signal plane 10 and the mounting pad 14a electrically connect with the ground plane 20 through the via 16a. If the electrical device 18 must be provided 3 V, 2 V, and 1 V of working voltage from the mounting pads 14b, 14c, and 14d respectively, the mounting pads 14b, 14c, and 14d must respectively individually connect with the power blocks 32c, 32b, and 32a. The mounting pad 14b electrically connects with the power block 32c through the trace regions 12, the via 16b of the signal plane 10, and the via 26b of the ground plane 20. Similarly, the mounting pad 14c electrically connects with the power block 32b through the trace regions 12, the via 16c of the signal plane 10, and the via 26c of the ground plane 20. Finally, the mounting pad 14d electrically connects with the power block 32a through the trace regions 12, the via 16d of the signal plane 10, and the via 26d of the ground plane 20. Additionally, the via 34 of the power plane region 30 can be used to transmit an input signal of the electrical device 18 of the signal plane 10, the input signal passing through the ground plane 20 and the power plane region 30 to connect with another signal plane located under the power plane region 30. Thus, a multi-layer printed circuit board according to the prior art can use plural signal planes 10 to transmit signals. Additionally, signal transmitting routes among electrical devices can be put on different signal planes 10. This can greatly reduce complexity over trace regions that are put on the same signal plane 10 of a single layer printed circuit board.
When complex transmitting signals among electrical devices is required, the construction of a multi-layer printed circuit board can use plural signal planes to define a complex trace regions layout. For example, typical computer systems are constructed of a plurality of electrical devices. The main computer devices, such as a CPU, a north bridge chip, and a memory, are mounted on the motherboard. For connecting these computer devices, the motherboard exploits the aforementioned multi-layer printed circuit board construction to solve complex trace routes among the computer devices. Because of cost, motherboards are typically four-layer boards including one first signal plane, one ground plane, one power plane region, and one second signal plane. The first signal plane includes trace regions and mounting pads, the second signal plane includes trace regions related to the first signal plane, the ground plane provides a ground reference level, and the power plane region provides plural voltage levels to support the computer devices.
Referring to FIG. 3, the insulating line 24 bounds the power blocks 32a–32c. Because the insulating line 24 is not a smooth curve, the boundaries of adjacent the power blocks 32a, 32b, and 32c include acute angles. Since adjacent power blocks 32a–32c provide different voltage levels, there is a dropout voltage among the adjacent the power blocks 32a–32c. Where the boundary forms acute angles there can exist a phenomenon of peak electrical discharge. This causes voltage levels of the power blocks 32a–32c to fluctuate. The power blocks 32a–32c working voltages to the computer devices. When these working voltages are not stable, fluctuation of signals at the high and low levels affect system stability.